Design Concept of Fiber Optic Bending Sensor using Incohrent Optical Frequency Domain Reflectometry and Two Photon Absorption Process in Avalanche Photodiode

In various fields, bending sensors based on optical fibers have gained significant attention. These include structural health monitoring and shape measurement of medical devices such as catheters. The characteristics of fibers, such as thinness, flexibility, light weight, and immunity to electromagnetic interference, make them suitable for various applications. Among various techniques, fiber Bragg grating (FBG) sensors are widely employed to measure strain through shifts in their reflection spectra, which are proportional to the applied strain. A single FBG can only measure the strain along one direction. In contrast, multicore fibers with Bragg gratings inscribed in multiple cores enable directional bending sensing by comparing the spectral shifts of peripheral cores against the central core. However, it is challenging to simultaneously measure overlapping reflection spectra from multiple cores using a single photodetector. Conventional approaches employ broadband light sources or wavelength-tunable lasers with multiple photodetection units, resulting in complex systems. In this study, we propose a novel method for spectral separation and identification that utilizes incoherent optical frequency domain reflectometry (I-OFDR) and the two-photon absorption (TPA) process of a silicon avalanche photodiode (Si-APD). I-OFDR uses laser light whose intensity is modulated by a chirped signal. The TPA photocurrent enables I-OFDR signal detection without the need for complex electrical circuitry. A key advantage of the proposed system is its fast measurement capability: in our proof-of-concept experiment, the measurement was completed in 25 s, which is significantly shorter than the several minutes typically required by our conventional methods. The system design and preliminary experimental results validate the feasibility and efficiency of the proposed approach.

Bending sensor; Fiber sensor; Incoherent optical frequency domain reflectometry; Two-photon absorption

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